1. Technical Field
The present invention relates generally to gas turbine engines having centrifugal compressors and, more specifically, maintaining forward thrust on a centrifugal compressor impeller.
2. Background Information
Positive forward rotor thrust is critical to the operation of a centrifugal compressor gas turbine engine. Maintaining a positive forward thrust on the impeller, or “rotor thrust” as it is often referred to, helps minimize the clearances between the shroud and blades of the impeller. Minimizing these clearances increases fuel efficiency and is often useful or necessary to satisfy required fuel efficiency specifications. Additionally, sufficiently small clearances must be maintained between the shroud and blades of the impeller in order to minimize losses between the tips of the blades and the shroud and to maintain sufficient stall margin. It is also important to avoid the rotor thrust to crossing over into the negative rotor thrust regime which could damage the engine. The resulting deflection of the overall rotor including the rotating hardware in the gas generator turbine, where tight clearances are maintained, could result in a damaging rub between rotating and stator hardware.
It is known in the art to minimize clearance between the blade tips of an impeller rotating within a gas turbine engine and a surrounding blade tip shroud to reduce leakage of a working fluid around the blade tips of centrifugal compressor stages. It is known that rotor thrust may be controlled by proper design of an inner radius of a swirl plate along an impeller backwall, which has only limited forward rotor thrust capability. A radial static pressure gradient along the impeller backwall exists as a result of windage losses between the rotor and stator. The precise design of the swirl plate inner radius results in a specific static pressure and piston area in which the impeller backwall bleed area provides forward pressure on the impeller, thus, positive forward rotor thrust.
It is known that increasing the inner radius of the swirl plate results in less windage losses and higher air static pressure in the cavity aft of the impeller as well as increased piston area aft of the impeller and, thus, increased forward rotor thrust. However, with this configuration, there exists a practical limit on how much forward rotor thrust can be achieved due to the limitations on how high the inner radius of the swirl plate can be designed. This capability of increasing forward rotor thrust by increasing the swirl plate inner radius is even more limited in the case where clean air from the impeller is used for turbine cooling since a windage shield would be necessary between the rotor and static inner combustor case.
Conventional engines employ clean air bleed systems to cool turbine components in gas turbines using an axi-centrifugal compressor as is done in the General Electric CFE738 engine. The turbine cooling supply air exits the centrifugal diffuser through a small gap between the diffuser exit and deswirler inner shroud. This air is then ducted radially inward by expensive integrally cast passages to the inside of the inner combustion case where it is then ducted into an accelerator via an arduous path where the airflow must make several 90 degree turns generating losses (and thus raising the temperature of the cooling air) before going through the accelerator. After leaving the accelerator, this cooling air travels up along a stage 1 turbine disk into a stage 1 turbine blade.
Thus, there continues to be a demand for advancements in impeller or rotor positive thrust control to maintain proper impeller blade tip clearance technology and provide efficient turbine cooling air from the impeller.